In today's microelectronic IC industry, semiconductor devices are miniaturized to micron (10−6) or even nanometer (10−9) scale and, as such, are built on the very top surface layer of a semiconductor substrate or wafer. The bulk of the substrate below the top device layer serves little purpose other than being a physical support structure or a heat sink. In fact, electrical and electronic coupling between the top device layer and the bulk can be, in some instances, quite deleterious.
In SOI technology, the SOI layer and bulk substrate layer are separated by a continuous insulating layer referred to as a buried oxide (BOX). This separation or isolation of the device layer (i.e., the SOI layer) can result in significant benefits and performance improvements including, for example, less junction capacitance and leakage; greater resistance to ionizing radiation, electrical noise and heat; immunity to CMOS latch-up; and etc. However, forming SOI structures is no simple matter.
After decades of research and development only a few methods are proven to be commercially viable. In one, called BESOI (bond-and-etch-back SOI), two Si wafers are oxidized at the surface and the oxidized surfaces are bonded together and then one of the two bonded wafers is etched to provide a thin SOI device layer. In this prior art method and its variations, as the wafer surfaces are oxidized before bonding, the buried oxide can be made to have any desired thickness. However, impurities at the bonded interface and the difficulty in achieving a thin, uniform Si over-layer through the etch-back process are major drawbacks. The terms “Si over-layer” and “SOI layer” may be interchangeably used in this application.
In another well-known method, called SIMOX (separation by implantation of oxygen), a selected dose of oxygen ions is directly implanted into a Si wafer, and then the wafer is annealed in an oxygen ambient at a high temperature so that the implanted oxygen is converted into a continuous buried oxide layer. The thickness of the buried oxide layer in the SIMOX method is mostly dependent on the implanted oxygen dose and the thermal oxidation conditions. Moreover, in SIMOX, the Si over-layer is thinned to a desired thickness during the thermal oxidation, after which the surface oxide is stripped off.
Normally, a 3E17 cm−2-5E17cm−2 level of oxygen implantation dose is required in SIMOX to form a low-defect, continuous buried oxide layer that separates the Si over-layer from the substrate. In order to facilitate the implantation of this high level of oxygen ions in a reasonable period of time, high-current implanters are specifically built for SIMOX application at an extra cost. With the Si substrate scaling up to larger wafer sizes, the high-cost of scaled-up high-current implanters and the implantation process itself is becoming a serious issue.
Porous Si is formed by electrolytic anodization in an aqueous solution containing HF. An HF-resistant electrode, such as one made of platinum, is biased negatively, and a lightly or heavily p-doped Si substrate is biased positively. The porosity, measured in terms of the volume loss, of the resulting porous Si layer formed in the surface of a Si wafer is proportional to the electrical current and voltage and inversely proportional to the HF concentration. The depth of the porous Si layer formed is proportional to the anozidation time for a given dopant concentration. The actual structure of the porous Si, however, is a very complicated function of the type and concentration of dopants and defects, in addition to the above-mentioned parameters. A common characteristic of porous Si materials is the enormous surface area associated with high-density pores: The surface area per unit volume is estimated to be 100-200 m2/cm3. The presence of this large surface area makes porous Si very susceptible to chemical reaction with an ambient gas such as oxygen. The oxidation rate of porous Si is found to be an order of magnitude higher than that of bulk Si. This makes porous Si a good candidate for oxide isolation.
In a well-known method, called FIPOS (full isolation by porous oxidized silicon), porous Si is formed, by using a patterning procedure followed by HF-anodization, to surround shallow Si islands, and then the porous Si is oxidized to provide Si islands of full isolation. Although the oxidized porous Si provides good isolation, it is typically too thick relative to the Si islands and laterally non-uniform. This non-uniformity leads to warping of the Si wafer and formation of many oxidation-induced defects in the Si islands.
In view of the above drawbacks with the prior art methods of fabricating SOI substrate structures, there is a continued need for providing a new and improved method of forming SOI substrate structures that is relatively simple and inexpensive.